1. Field of the Invention
The present invention relates to a zoom lens, and particularly relates to a compact zoom lens for use in an image projection optical system of an image projection device and also for use in an image pickup optical system of an image pickup device. The present zoom lens offers a long back focal length and high-resolution combined with high image brightness with a compact package, and is particularly suitable for use in a mobile image projection device.
2. Description of Prior Art
As an important element of an image projection optical system or an image pickup optical system, optical lenses are generally required to satisfy the high zoom ratio, high-resolution and high image quality requirements. A high zoom ratio lens generally consists of a plurality of lens groups and a considerable number of constituent lens elements, whereby the overall length of the lens is rather long. To meet the additional high-resolution requirement, special low dispersion lens elements and aspheric lens elements are Her incorporated in the lens. Conventional zoom lenses are generally designed for image pickup purposes, and accordingly the back focal length of the zoom lens is normally short but can meet the requirements of the image pickup device. However, such a conventional zoom lens cannot be applied to an image projection device due to its short back focal length. For example, for a liquid crystal projector, a projection zoom lens used therein is required to have a long enough back focal length to provide room for a color separating and synthesizing optical system of the projector and also to satisfy the high resolution requirement conforming to the high resolution of the liquid crystal panels of the projector. Further, high image quality with enhanced image brightness is often required for a zoom lens. As a rule, the smaller the F-number (expressed as the focal length of the lens divided by the effective aperture diameter), the brighter the lens. A single lens usually has a small F-number, whereas a zoom lens usually has a large F-number, so the view appears dark and manual focusing becomes difficult. Accordingly, a zoom lens with a small F-number is desired for enhanced image brightness and hence better image quality. To satisfy all these requirements as discussed above, various zoom lenses for use in an image projection optical system have been proposed, for example, in U.S. Pat. Nos. 6,590,716 and 5,619,381.
U.S. Pat. No. 6,590,716 discloses a conventional projection zoom lens that comprises four lens groups including twelve or thirteen constituent lens elements. The overall focal length of this projection zoom lens is variable between approximately 28 mm at the wide angle end and a value between 37 mm and 38 mm at the telephoto end. The F-number is 1.7 to 2.0. This conventional projection zoom lens effectively reduces various aberrations and improves the quality of a projected image. However, there is recently not only a demand for an image with high resolution but also a demand for reduction in size and weight of the projector with importance attached to maneuverability and portability of the projector with a small panel. Compact projectors have been found to be incorporated into notebook computers, desktop computers, DVDs, and so on. Therefore, this conventional projection zoom lens is not fit for use in a compact projector because an optical system employing the four-group zoom lens, which has an overall focal length between 28 mm and 38 mm, will be bulky. Further, The F-number of this conventional projection zoom lens, which is 1.7 to 2.0, may also not satisfy the requirement for a bright image in some cases.
A compact two-group movable lens is disclosed in U.S. Pat. No. 5,619,381. This compact two-group lens includes a zoom lens group having a negative refractive power and a compensator group having a positive refractive power. Each group contains at least one aspheric lens element for elimination of aberrations. A projection optical system employing this movable lens has a low distortion and high resolution with fewer lens elements. However, this patent only roughly introduces the configuration and the function of the two-group movable lens, and does not provide any detailed description of conditions and parameters necessary to the design.
To obtain the desired zoom ratio, image resolution and optical performance within the entire zoom range, the lens configuration for each lens group of a projection zoom lens must be carefully designed. When the refractive power of the lens group is increased, the movement stroke of the lens group during zooming will be shortened and thus the overall length of the zoom lens can be reduced. However, the increase in refractive power of the lens group will bring the problem of aberration fluctuations during zooming, and these aberration fluctuations are difficult to correct.
Accordingly, how to reduce the size and weight of the whole zoom lens system while maintaining high optical performance is a problem encountered by a zoom lens designer.